Simplify the following expression: $a = \dfrac{3r}{4} + \dfrac{9r}{10}$
In order to add expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $4$ and $10$ $\lcm(4, 10) = 20$ $ a = \dfrac{5}{5} \cdot \dfrac{3r}{4} + \dfrac{2}{2} \cdot \dfrac{9r}{10} $ $a = \dfrac{15r}{20} + \dfrac{18r}{20}$ $a = \dfrac{15r +18r}{20}$ $a = \dfrac{33r}{20}$